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''Asplenium pinnatifidum'', commonly known as the lobed spleenwort or pinnatifid spleenwort, is a small fern found principally in the Appalachian Mountains and the Shawnee Hills, growing in rock crevices in moderately acid to subacid strata. Originally identified as a variety of walking fern (''Asplenium rhizophyllum''), it was classified as a separate species by Thomas Nuttall in 1818. It is believed to have originated by chromosome doubling in a hybrid between walking fern and mountain spleenwort (''Asplenium montanum''), producing a fertile tetraploid, a phenomenon known as alloploidy; however, the hypothesized parental hybrid has never been located. It is intermediate in morphology between the parent species: while its leaf blades are long and tapering like that of walking fern, the influence of mountain spleenwort means that the blades are lobed, rather than whole. ''A. pinnatifidum'' can itself form sterile hybrids with several other spleenworts. ==Description== ''Asplenium pinnatifidum'' is a small fern with bright green, wrinkled, pinnatifid (lobed) fronds. These form evergreen, perennial tufts. Notable characteristics are the shiny stem, dark only at the base, and the long-tapering, variably lobed leaf blades. The fronds are monomorphic, the sterile and fertile fronds appearing the same size and shape. The roots of ''A. pinnatifidum'' are not proliferous, so it appears as clusters of leaves springing from a single rhizome. The leaves are closely spaced on the rhizome, which is frequently branched. The rhizome is about in diameter, covered with narrowly triangular scales which are dark reddish-brown or blackish in color, and strongly clathrate (bearing a lattice-like pattern). The scales are long and 0.3 to 0.5 millimeters wide, with untoothed edges. The stipe (the part of the stem below the leaf blade) is shiny and dark reddish brown at the base. The color fades to green in the upper one-third to one-half of the stipe. It is covered in narrowly triangular, dark reddish-brown scales at the base, which diminish into hairs in the upper part of the stipe. It may show narrow wings from the base of the leaf to near the base of the stipe. The stipe is long, and may be from one-tenth to one and one-half times the length of the blade. The overall shape of the blade is narrowly triangular or lance-shaped, sometimes with an irregular outline. The blade tapers to a long point, the length of the taper being variable among specimens. The blades are generally curled with downward-pointing tips. On very rare occasions, buds are found at the tip, but these are not known to form new plants in nature. The base of the blade may be squared off or notched to a varying extent along the rachis (central axis of the leaf). The blade ranges from long, rarely to , and in width, rarely to , and is thick and somewhat leathery. Blades are either entirely pinnatifid (lobed but not completely cut), or cut to form a single pair of pinnae at the base. When they exist, the pinnae are roughly oval or triangle-shaped, sometimes narrow, and are from long, rarely to , and 0.4 to 1 millimeters in width (rarely to 1.2 millimeters). The base of the pinnae may be squared off or taper to a point, while the edges are wrinkled to toothed. The tip can vary from rounded to pointed. The lobes of the blade gradually diminish towards the tip, sometimes becoming simply wavy. The rachis is green, sometimes turning tan when dry. The underside of the rachis and blade have a few scattered, small hairs. Overall, the blades are quite morphologically variable; in younger blades, the edges may be not at all lobed or may be wavy. The veins are free and forking, only rarely anastomosing (rejoining one another to form nets). Each segment (pinna or lobe) of a fertile frond has one to six sori, sometimes more than forty in extreme cases. The sori usually fuse with one another as they age. These are long They are covered with thin, whitish indusia with untoothed edges, which are persistent. Each sporangium holds 64 spores. The species has a chromosome number of 144 in the sporophyte, indicating an allotetraploid origin. ''A. pinnatifidum'' is somewhat similar to its parent species ''A. rhizophyllum''. In comparison, however, ''A. pinnatifidum'' is distinctly lobed when mature, tends to have longer stipes in proportion to its leaf size, and has a more upright habit. It might be confused with Countess Dalhousie's spleenwort (''A. dalhousiae''), of Asia and the American Southeast, but the latter has short, dull stipes with larger, toothed scales. ''A. pinnatifidum'' closely resembles the hybrid Scott's spleenwort (''A. × ebenoides'') (including the fertile Tutwiler's spleenwort, ''A. tutwilerae''), but those species have a wholly dark stipe, with the dark color extending into the rachis, and longer lobes on the blade. Among the hybrid species of which it is a parent, ''A. pinnatifidum'' is most similar to Graves' spleenwort (''A. × gravesii''), a hybrid with Bradley's spleenwort (''A. bradleyi''), and to a lesser extent, to Trudell's spleenwort (''A. × trudellii'') and Kentucky spleenwort (''A. × kentuckiense''). In ''A. × gravesii'', the dark color of the stipe extends to the base of the leaf blade, the blades often have more than one pair of pinnae, and their edges are shallowly wrinkled or toothed. In addition, the basal pinnae, which may themselves be pinnatifid, lack a stalk, the leaf blade is pointed at the tip but not drawn out at length, and there are generally fewer fronds. Its sori are dark brown, rather than cinnamon brown. ''A. trudellii'' is fully pinnate in the lower half of the blade, and its pinnae are toothed. ''A. × kentuckiense'' is also fully pinnate towards the base of the blade, with four to six pairs of pinnae, and the brown color of its stipe extends up into the basal part of the rachis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Asplenium pinnatifidum」の詳細全文を読む スポンサード リンク
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